I think there are a few forum threads out there regarding this issue and I think this is due to the Browser placing the cursor in the ComboBox. In order to get rid of it you will have to disable the input of the RadComboBox (which prevents you from typing in it).
You could try setting AllowCustomText property of the RadComboBox to false which prevents typing any text in the input of the RadComboBox. Also set the EnableLoadOnDemand and MarkFirstMatch
properties to false and see if it helps.